Tanning of hides and skins



Patented Dec. 31, 1940 PATENT OFFICF.

TANNING F HIDES AND SKINS Robert L. McGrath, Salem, Mass., assignor to The Tanning Process Company, Boston, Mass, a corporation of Massachusetts No Drawing.

Application October 17, 1938 Serial No. 235,453

Claims.

This invention relates to the tanning of hides and skins and more particularly to the tanning of the lighter skins such as calf, kid, and sheepskins to produce a white leather. While the invention is described with reference to a combination chrome and silicate tannage of sheepskins to provide a chamois-like leather, it is to be understood that the invention and various characteristics thereof may have other applications and 0 uses.

Heretofore, white chrome-tanned leathers have been made by methods which involved drumming into the tanned skin such white powders as flour, talc, and kaolin. For example, in the making of white chrome-tanned sheepskin leather, the common practice is to bleach the tanned skins with skins to oxidation and fermentation either in hot chambers, or in piles, where they are allowed to heat for a suitable length of time. The heated skins have to be watched carefully as there is danger of overheating. Subsequently the skins are soaked in warm water and then placed in a hydraulic press to remove the uncombined oil.

. The method outlined above requires considerable time and the attention of an expert. The result is that chamois of good grade brings relatively high prices.

It is an object of this invention to produce a soft, white or colored leather which will be waterabsorptive and have many of the qualities of the best grades of chamo-is, while being much less.

expensive. It is a further object 'of the invention to provide a method of tanning to produce white leathers, which method may be readily practised by the ordinary tanner, in a fraction of the time heretofore required in production methods for this type of leather.

It is an important characteristic of the invention that the skins are subjected to treatment by a combination of a silicate of an alkali metal and a chromium salt, in an acidified solution. By the term chromium salt, I mean a salt containing chromium and useful in tanning operations upon hides and skins. Most satisfactory results have been secured by acombination of sodium silicate and chromium sulphate in a solution acidified by sulphuric-acid. By the use of this preparation actual tanning operations in the drum consume only about two hours. The resulting product is an unusually white leather made without loading the leather or covering a surface thereof with flour, chalk, clay, or other extraneous matter.-

In the manufacture 'of a soft chamois-like leather, a layer is cut from the denser firmer grain surface of the skin before it is tanned 'as above described. Subsequently it is fatliquored and, when dry, buffed on both sides. Removal of the denser portion of the grain surface of the skin, together with the buffing of both surfaces of the skin conduce to the making of a soft leather from a tannage which also contributes to that end.

In the practise of this method for making soft leather with chamois-like qualities, sheepskins which have been depilated are subsequently limed, hated and pickled. Following the pickling operation the skins are degreased while in a wet condition and are then washed in fresh running water in orderto remove the pickle and salt solution and to swell the skins. Depending upon the amount of pickle in the skins after the degreasing operation, this washing in fresh water will consume from fifteen minutes to seventy-five minutes, the object being to secure a swelling of the skins such that the grain surface may be readily removed, at the same time avoiding loss of substance such as would be caused by excessive hy-' dration of the skins. Following the washing operation, the skins are drained for about tw' hours. At this stage of operations the skins should be slightly swollen and not slippery.

The next important step is to remove a substantial layer of the grain surface of the true skin, this being conveniently accomplished by running the skins through a fieshing machinewhose feed roll speed has been slowed down about one t ird. The purpose here is to secure a surface which will be similar to the flesh surface of the skin but substantially finer in its fibrous appearance since in the grain side of the skin the fibers are finer and more densely packed together than on the flesh surface. 1

After the removal of a layer from the grain surface, the skins are put back into the drum and drummed for thirty minutes in brine of Bkr., using about six gallons of the brine per dozen skins, the purpose being to' deplete the skins, i. e., reduce their swollen condition to the normal for tanning operations.

As a preliminary to the tanning operation these skins are put into a drum to have their pH value raised. For this purpose ten dozen sheepskins with a pH of about 3.5 are placed in a drum and drummed with ten quarts of brine of 60 Bkr. and five gallons of water at a temperature of 70 F. for about ten minutes, after which there is added to the drum about ten pounds of sodium acetate crystals in five gallons of water at 80 F. Following this addition of the acetate, drumming is continued for about twenty minutes. Upon testing it will be found that the pH of the skins is now about 5. The skins are now ready for the tanning solution.

The following tanning solutions have been found satisfactory when made up in the proportions indicated:

Solution C.-20 ozs. basic chromic sulphate dissolved in 2 /2 gals. water.

For the treatment of ten dozen skins which have received the necessary preliminary treatment above described, there is provided twenty gallons of each of Solutions A and B and twelve and one-half gallons of Solution C. About onehalf of this mixture is placed with the ten dozen skins in the drum and the latter is driven for about thirty minutes after which the second half of the mixture is added and the drumming continued for another ninety minutes. A test of the skins at this time will show a pH value of about 4.35; These skins are then removed'from the drum and horsed up overnight. In the morning it will be found that they will stand the boiling test, that is, immersion in water at 212 F. for

two minutes, showing that they are thoroughly tanned.

While sodium silicate has been found to give eminently satisfactory results, it is to be understood that the corresponding potassium salt may be used in the described combination. Furthermore, a chromium salt other than the basic sulphate may be employed although the results are not apt to prove so satisfactory. For example,

. the chrome element in the combination described above may be secured by the use of either the normal or the basic chromium chloride. However, these salts give relatively scant tannages as compared to that produced by the chromium sulphate.

In making a mixture of Solutions A, B and C, the following directions are to be carefully observed to secure the best results. First mix Solutions B and C and then pour into this mixture Solution A very slowly and with constant stirring. It is important that the solutions be mixed in this order, otherwise the silicic acid would be precipitated and the whole solution would form a gel. The twenty-eight fluid ounces of sulphuric acid is the amount calculated to give a slight amount of acid in excess of that needed to change the basic chromium sulphate to the normal chromium sulphate and to neutralize the alkali in the sodium silicate. The pH of the tanning solution is about 2.4, which is decidedly on the acid side.

After the skins have drained overnight they are prepared for fatliquoring by treatment with a neutralizing agent such as sodium bicarbonate. For the ten dozen skins use about sixteen ounces of sodium bicarbonate in twenty-five gallons of water at 100 F., the drumming operation being continued for about twenty minutes. Then wash the skins with water at 70 F. for twenty minutes and then with the water at 140 F. for about five minutes.

The skins are now ready for fatliquoring which may be conveniently carried out by treating them with sulphonated neatsfoot oil of a good grade, about three hundred and eighty-four fluid ounces mixed with thirty-two ounces of Duponal Flakes in ten gallons of water at 150 F., the drumming being continued for about thirty minutes. The Duponal preparation mentioned above is a fatty alcohol sulphate useful as an emulsifying agent in fatliquoring operations on this special tannage.

Following the fatliquoring operation skins are slicked out and hung up to dry. When sufficient- 1y dried they are staked and then bufi'ed on both sides. This leather will be found to be soft like chamois and white in color. If it is desired to give the skins a chamois-like appearance they may be dyed, utilizing the following formula for ten dozen skins:

For ten dozen skins:

Dupont Leather Orange R ozs 2 Dupont Metinil Yellow Conc -ozs 1.6 Water at 150 F gals 10 drumming for about 20 minutes. Subsequently there is added twenty ounces of formic acid in ten gallons of water of a temperature of 100 F., the drumming being continued for another twenty minutes. As a result of the dyeing operation the skins will appear a light yellow, quite similar to the usual chamois leather.

This leather closely resembles oil-tanned chamois leather. It absorbs water readily and, therefore, can be used for many of the purposes of chamois, such as washing windows and automobiles. Since it is a soft washable leather, it may also find use in the making of inexpensive washable gloves for women. By a washable leather is meant a leather which will undergo washing and drying without substantial change in appearance and feel, whereas mineral tanned leathers are commonly changed inappearance by such operations and feel harsh and crusty after drying. It possesses an advantage over chamois in that it will stand water of a temperature up to about 200 F. whereas oil-tanned chamois will be spoiled if placed in water over 140 F.

Having described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. That improvement in methods of making a soft chamois-like leather from sheepskins which comprises subjecting degrained sheepskins to an acidified solution, containing a silicate of an alkali metal and a chromium salt in the proportion of several parts of the alkali silicate to one part of the'chromjum salt, for a period of less than three hours.

2. That improvement in methods of making a soft chamoislike leather from sheepskins which comprises subjecting degrained sheepskins to a solution capable of producing in the skins a pH value of about 5, and then treating the skins with an acidified solution, containing a silicate of an alkali metal and a chromium sulphate in the proportion of about six parts of the alkali silicate to one part of the chromium sulphate, for a period of about two hours.

5 3. A tanning composition comprising silicate of sodium about 128 parts, basic chromium sulpha'te about 20 parts, and a strong acid suflicient in quantity and effective to produce an acidified skin free from its grain layer and in combination with the reaction products of a chormium salt and a soluble silicate in an acidified solution, said leather being characterized by being soft, water absorptive and washable, and being capable of withstanding immersion for several minutes in hot water up to about 200 F.

ROBERT L. McGRATH. 

